§ Open mapping theorem
- Given a surjective continuous linear map f:X→Y, image of open unit ball is open.
- Immediate corollary: image of open set is open (translate/scale open unit ball around by linearity) to cover any open set with nbhds.
§ Quick intuition
- Intuition 1: If the map f we bijective, then thm is reasonably believeable given bounded/continuous inverse theorem, since f−1 would be continuous, and thus would map open sets to open sets, which would mean that f does the same.
- In more detail: suppose f−1 exists and is continuous. Then f(U)=Vimplies (f−1)−1(U)=V. Since f−1 is continuous, the iverse image of an open set ( U) is open, and thus V is open.
§ Why surjective
- Consider the embedding f:x↦(x,x) from R to R2.
- The full space R is open in the domain of f, but is not open in R2, since any epsilon ball around any point in the diagonal (x,x) would leak out of the diagonal.
- Thus, not every continuous linear map maps open sets to open sets.
§ Proof